1. Technical Field
The present invention relates to food products and to their methods ot preparation. More particularly, the present invention relates to edible food coating compositions comprising cross-linked shellac and to their methods of preparation.
2. The Prior Art
Although the coating of food to protect such food against oxidative degradation, microbial attack, and moisture penetration is well known, most coatings employed for such purposes are not edible and must be removed before the food can be consumed. If the coating employed adheres well to the the food product, the removal of such coating can be difficult and time consuming. Additionally, if the food product is brittle and fragile, the food product can break during the stripping of the coating, making the consumption of the food difficult and resulting in the loss of the food product. An edible food coating which does not require removal prior to consumption of the food product and which affords the necessary protection, particularly against moisture penetration, is therefore highly desirable.
A particular problem exists in the protection of food products with edible coatings or barriers with respect to composite foods comprising phases of dissimiliar materials, e.g., ice cream sandwiches or cheese and crackers, whose composite phases may differ in such properties as water activity, acidity, protein level and the like. Due to the various gradients in water activity, (as described in "Moisture Sorption," T. P. Labuya, American Association of Cereal Chemists, St. Paul, Minn. 1984) and the like between the phases as well as the physical contact, migration and/or diffusion of species between the phases can occur which can result in degradation of the properties of each phase. For example, moisture may migrate from the cheese to the cracker undesirably drying the cheese and at the same time undesirably reducing the crispness of the cracker. Furthermore, removal of any intermediate barrier material can be quite inconvenient.
Among the various potential gradients in such composite food articles, moisture migration remains the most significant problem area. While throughout the remainder of the specification below, particular attention is addressed to the problems of moisture migration and moisture penetration of coating or barrier compositions, the skilled artisan will appreciate that the present invention also finds usefulness in the problems associated with additional migration or penetration problems including oxygen, acidity, flavor, color, oil and protein.
In the past, the art has attempted to prepare composite food articles by formulating the different food phases so that the water activity of the phases were approximately the same so as to minimize moisture migration. However, not only does this limit the range of composite food products possible, but moisture migration and solute diffusion problems remain nonetheless. One approach towards providing an edible, low water permeable barrier has been to formulate barriers based upon compound fats and formed in-situ gel membranes (as described in U.S. Pat. No. 4,396,633, issued Aug. 2, 1983 to Tresser and U.S. Pat. No. 4,401,681, issued Aug. 30, 1983 to Dahle). However, such barrier or coating compositions suffer from numerous disadvantages. Among the problems is that in order for compound fat coatings or barriers to be effective over long periods of time, the fat coatings must be relatively thick. Additionally, especially with regard to chilled or frozen food articles, the fat barriers become relatively brittle at these reduced temperatures. Fissures or cracks may occur breaching the integrity of the barrier and allowing moisture migration to occur. Also, the fat coatings may be organoleptically undesirable providing a noticeable presence in an undesirably waxy mouthfeel especially at reduced consumption temperatures.
The prior art additionally includes attempts at providing edible coating compositions of low water permeability which are effective as relatively thin films. Coating compositions based upon modified methyl or ethyl cellulose ether are known (see U.S. Pat. Nos. 3,471,304 and 3,471,303, each issued Oct. 7, 1969 to M. M. Handy et al.). Additionally, coating compositions based upon shellac are also known (see U.S. Pat. No. 3,741,795, issued June 26, 1973 to C. A. Signorino). However, the compositions of each of these two references suffer from several disadvantages. First, each of the food compositions, while designated as edible, typically include ingredients which are not approved by the Food and Drug Administration. Additionally, while the coating compositions may provide a measure of water impermeability, further improvements in this important attribute are desirable. Also, the '304 patent teaches the necessity of employing both acylated fatty glycerides and certain metal salts.
The present invention resides, in part, in the preparation of new edible food coating compositions by incorporating into the molecular structure of shellac resin, a hydroxypropyl cellulose, ("HPC" is a cellulose ether containing propylene glycol groups attached by an ether linkage which contains, on an anhydrous basis, not more than 4.6 hydroxypropyl groups per anhydro glucose unit) and hydroxypropyl methyl cellulose, ("HPMC" is a cellulose ether containing propylene glycol and methyl groups attached by an ether linkage) either singly or in combination and with or without the addition of an acid catalyst.
The invention is suitable for commercial operations and provides compositions which have improved properties in the edible food coating fields. The compositions of the present invention may be prepared in the form of pre-polymerized or baked-on films depending on the heat tolerance properties of the substrate coated. After air drying or curing the film possesses excellent oil, water and aging resistance and unusual toughness and elasticity or flexibility. Various other outstanding properties will be apparent from the following description of the compositions of the present invention disclosures.
When small molecules diffuse through a polymer membrane, the rate of permeation can be expressed by parameters which may be characteristic of the polymer. The general concept of the ease with which a permeant passes through a barrier or the ease with which an intact material can be penetrated by a given gas or liquid is often referred to as "permeability." This general term "permeability" does not refer to the mechanism or imply anything about the mechanism of the permeation but only to the rate of the transmission or transport.
Membranes or films are generally described as permeable, semi-permeable (permeable to some substances but not to others), or perm-selective (permeable to different extents to different molecular species under equal driving force). Consequently, a given membrane may be described by any one of these terms depending upon the nature of the penetrant or penetrants being studied (e.g., cellulose is permeable to water, perm-selective to water-glucose solutions and semi-permeable to water-protein solutions).
The terms permeability and permeability coefficient are defined in various ways by different authors, particularly when they are involved in different areas of research. The skilled artisan obtaining permeability information from the literature must therefore look carefully at the units of the permeability constants and the method of measurement. The permeability coefficient P is generally the proportionality constant between the flow driving force per unit thickness of membrane. In the literature, however, one also finds flow per time, flow per area per time, or flow per area per time per unit thickness, all under the general term permeability. In the latter cases, the permeability coefficient may be an intrinsic property of the membrane, or it may be only a phenomenological property dependent on experimental conditions during measurements.
For purposes of illustration in Table 1 below, the permeability to water vapor or, synonymously, the water vapor permeability constants of a number of different non-edible polymer films are listed (as reported in "Permeability of Plastic Films and Coated Paper to Gases and Vapor," V. Stannett, et al., TAPPI Monograph Series No. 23, 1963, and "Polymer Handbook," H. Yasuda, and V. Stannett, John Wiley and Sons (Interscience Division), New York, III-229-240, 1975.), and serves as a guide to the current capabilities of overwrap packaging film water vapor permeability.
TABLE 1 ______________________________________ Water Vapor Film Thickness* Permeability** ______________________________________ Low density polyethylene .0022 7.3 High density polyethylene .0045 35 Polyvinylidene chloride .0025 0.05 (Saran) Polyacrylonitrile N.A. 30 Cellulose acetate N.A. 680 (unplasticized) Polystyrene N.A. 120 Ethyl cellulose N.A. 1200 ______________________________________ *thickness in inches **units [cm.sup.3 (STP)cm.sup.-2 sec.sup.-1 (cmHg).sup.-1 cm .times. 10.sup.-9 in the above table, while exemplary film thickness are given for illustration of typical use film thickness, the water vapor permeability values are intrinsic to the materials and independent of thickness.
Also for purposes of illustration, examples of several edible films not within the scope of the present invention comprising ordinary shellac, cellulose derivatives and/or simple mixtures thereof and other currently available food approved film formers or coatings are listed in Table 2 along with their water vapor permeability constants as determined by methodology outlined in ASTM E96-66 (Reapproved 1972).
TABLE 2 ______________________________________ Water Vapor Film Thickness* Permeability** ______________________________________ Hydroxypropyl methyl .002 49,280 cellulose Hydroxypropyl cellulose .002 873 Zein-Corn protein .0004 168 Paraffin wax on citrus .0011 10 fruit.sup.A,B. Shellac-bleached .0005 90 Shellac-unbleached .0012 81 Shellac-HPMC (heat treat) .0003 38 Shellac-HPC (heat treat) .0003 43 ______________________________________ *thickness in inches **units [cm.sup.3 (STP)cm.sup.-2 sec.sup.-1 (cmHg).sup.-1 cm .times. 10.sup.-9 .sup.A use limited by 21 CFR 172.275 (i.e., the Code of Federal Regulations, Vol. 21, .sctn.172.275) 1984. .sup.B W. M. Miller and W. Grierson, Transactions of the ASAE, 1884-1887, 1983.
Ordinary shellac and cellulose derivatives have been used in or as a glaze in the pharmaceutical and confectionary industries. Food grade shellacs and/or cellulose derivatives are dissolved in ethyl alcohol and used for coating tablets and confections by panning, spraying, brushing or curtain coating methods. However, pure cellulose derivatives generally are poor coatings as they impart a lubrious texture when dissolving, exhibit minimal flexibility and generally provide poor water impermeable coatings. Additionally, known coating compositions based upon pure bleached shellac suffer from other disadvantages as well. Noticeable off-flavors can be associated with shellac. While these problems are of less concern in the fabrication of orally administered medicines or vitamins, such problems are significant in other types of food products. Additionally, while known shellac based coatings initially otter good moisture impermeability, such coatings tend to swell in the presence of moisture over time. As the coatings swell and absorb moisture, the barrier properties deteriorate. Thus, there is a continuing need for improved shellac based coating compositions with extended shelf life due to decreased susceptibility to swelling in the presence of moisture. Accordingly, it is an object of the present invention to provide edible coating compositions with improved moisture impermeability.
It is a further object to provide coating compositions having improved resistance to water swelling.
Another object of the present invention is to provide coating compositions which contain neither non-food approved ingredients nor metal salts of fatty acids.
Still another object is to provide methods for preparing such coating compositions.
It is an object of the present invention to provide methods for preparing such coating compositions which can be used with heat-sensitive substrates.